Method of assembling transducer acoustically coupled by silicone grease

ABSTRACT

A transducer which includes an element assembly which is acoustically coupled to a boot by silicone grease, such as dimethyl polysiloxane oil and silicone oxide. A method of assembling the transducer may comprise coating an elongated element assembly with the silicone grease; inserting an elongated resilient boot into an evacuating tube which is open at one end and closed at an opposite end; drawing a vacuum inside the tube at a position intermediate its ends for expanding the boot tightly against the inner surface of the tube; inserting the coated element assembly into the expanded boot; wiping the silicone grease smooth on the element assembly as the assembly is inserted into the tube; releasing the vacuum inside the tube; and withdrawing the assembly and boot from the tube.

United States Patent [191 [5 METHOD OF ASSEMBLING TRANSDUCER ACOUSTICALLY COUPLED BY SILICONE GREASE [75] Inventor: Lawrence R. C. Ma, Honolulu,

Hawaii [73] Assignee: 1 The United States of America as represented by the Secretary of the Navy, Washington, DC.

22 Filed: Nov. 24, 1972 21 Appl.No.:309,579

[52] US. Cl 29/l69.5, 29/234, 29/235, 29/450, 29/458, 340/8 [51] Int. Cl 829d 17/00 [58] Field of Search..... 29/450, 594, 451, 458, 235, 29/234, 169.5; 340/8 1 Jan. 22, 1974 8/1962 Workman 291450 10/1971 Guenther et al 29/235 X [57] ABSTRACT A transducer which includes an element assembly which is acoustically coupled to a boot by silicone grease, such as di-methyl polysiloxane oil and silicone oxidev A method of assembling the transducer may comprise coating an elongated element assembly with the silicone grease; inserting an elongated resilient boot into an evacuating tube which is open at one end and closed at an opposite end; drawing a vacuum inside the tube at a position intermediate its ends for expanding the boot tightly against the inner surface of the tube; inserting the coated element assembly into the expanded boot; wiping the silicone grease smooth on the element assembly as the assembly is inserted into the tube; releasing the vacuum inside the tube; and withdrawing the assembly and boot from the tube.

6 Claims, 8 Drawing Figures PATENTH] JAN 2 2 I874 SHEEI 2 BF 2 METHOD OF ASSEMBLING TRANSDUCER ACOUSTICALLY COUPLED BY SILICONE GREASE STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates to a transducer such as acoustically coupled to a boot by silicone grease, and the method of construction thereof.

Many hydrophones include an element assembly which is substantially jacketed by an exterior rubber boot. For years, the US. Navy has been employing castor1oil as an acoustic couplant between the element assembly and the surrounding boot. Castor oil was originallyused asan acoustic couplant because it has a p.c. similar to water and it filled the interior cavities within the .early hydrophones. Few of the modern hydrophones have interior cavities which require filling, and are now constructed with a very thin film of acoustic couplant between the element assembly and the surrounding rubber boot. Because of the extremely thin filmof acoustic couplant compared to a wavelength of sound in the frequency range of interest for sonar operations p.c..matching became meaningless. In spite of the advance in the art castor oil continued to be used as the acoustic couplant. The use of castor oil has caused a considerable problem in the desired performance of Navy hydrophones. The reasons that castor oil has been unsatisfactory as a present day acoustic couplant are because it deteriorates with age and is absorbed by the rubber boot. The same problem also existed with active transducers.

SUMMARY OF THE INVENTION The. present invention overcomes the aforementioned problems by utilizing a silicone grease, such as di-methyl polysiloxane oil and silicone oxide, as an acoustic couplant between the transducer element assembly and the surrounding rubber boot. A method of assemblying such a hydrophone includes the steps of coating the element assembly with silicone grease; inserting a resilient boot into an evacuating tube which is open at one end and closed at the opposite end; drawing a vacuum inside the tube at a position intermediateitsaiends for expanding the boot tightly against the inner surface of the tube; inserting the coated element assembly. into the expanded boot; wiping the silicone grease smooth on the assembly as the assembly is inserted into the tube; releasing the vacuum inside the tubegand withdrawing the assembly and boot from the tube. .The present invention can be utilized for all transducers whether active or passive.

OBJECTS OF THE INVENTION An object of the present invention is to overcome the aforementioned problems associated with prior art transducers.

Another object is to provide a transducer which utilizes an improved acoustic couplant between its element assembly and surrounding rubber boot.

. a hydrophone, which has an element assembly which is A further object is to provide a method of constructing a hydrophone which utilizes silicone grease as an acoustic couplant between an element assembly and surrounding rubber boot.

Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention, when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exemplary hydrophone element assembly.

FIGS. 2 through 7 illustrate a method of constructing a hydrophone which utilizes silicone grease as an acoustic couplant between the element assembly and a surrounding rubber boot.

FIG. 8 illustrates a completed hydrophone with the boot banded to the element assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, there is shown in FIG. I a hydrophone element assembly 10 which includes a plurality of crystal elements 12 with separators l4 therebetween. End pieces 16 and 18 are centrally joined (not shown) for holding the entire assembly together. FIG. 8 illustrates a completed hydrophone 20 with the element assembly 10 jacketed by a resilient boot 22 which may be constructed of rubber. The boot 22 is held to the element assembly 10 by a pair of bands 24. Between the element assembly 10 and the boot 22 is a thin film acoustic couplant 25. In the past castor oil has been used as the couplant, however, because of deterioration and absorption by the boot the hydrophone 20 had a short usable life. The usable life was sometimes no more than 6 months even though the hydrophone was in storage and was never put into operational use. The short life of Navy hydrophones utilizing castor oil as an acoustic couplant has been a long standing problem in the art. To reduce this operational disadvantage the Navy has in the past been applying the castor oil as an acoustic couplant just prior to final testing and shipping of the hydrophone to the user.

The present invention overcomes the problems associated with the use of castor oil as an acoustic couplant by utilizing silicone grease, such as di-methyl polysiloxane oil and silicone oxide. This grease can be purchased from Dow-Corning under the trademark DC 111. The grease is a stiff, non-melting, material which maintains its consistency over a temperature range from 40 to 500 F. Because of its low volatility, chemical resistance, and sealing ability it is commonly used as a sealant for vacuum and pressure systems and as a sealant and lubricant for chemical processing equipment. It is also used in pressure lubricated plug valves, control valves, flow meter bearings, ceramic plug cocks, and the like because of its non-oxidizing and its non-gumming characteristics. It has outstanding water resistance and therefore finds its use for lubricating valves of water softeners. In electronic devices, the grease serves as both a damping and a dielectric medium. It is also used as a lubricant for synthetic rubber O-rings and seals in high temperature applications, thus preventing the rubber from adhering to shaft or cylinder applications. However, the grease was never known to be suitable as an acoustic couplant in hydrophone applications. In the present invention the grease is used r as an acoustic couplant between any element means of a hydrophone and its surrounding boot. In a comparison efficiency test between castor oil and DC 1 l l as an acoustic couplant DC I l l was found to be significantly superior. The relative efficiency in dB of DC 1 l 1 compared to castor oil, utilizing castor oil as a zero base, was found to be six to zero utilizing the following formula:

Relative efficiency 10 log (Pf/p2) where p denotes a received sound pressure or the voltage output from a hydrophone monitoring that pressure. With time, the castor oil will further deteriorate and the relative efficiencies stand out in greater contrast. It has been found that there is no deterioration whatsoever of the DC 1 1 1 when it is used as an acoustic couplant.

FIGS. 2 through 7 illustrate a method of assemblying the hydrophone 20 utilizing silicone grease as an acoustic couplant. In FIG. 2 there is illustrated an evacuating tube 26 which has an open end 28 and a closed end 30. To the left of the evacuating tube 26 is a tubular rubber boot 32 which has an exterior diameter which is slightly smaller than the interior chamber of the evacuating tube 26. The boot 32 is substantially the same length as the element assembly. As illustrated in FIG. 3, the boot 32 is partially inserted into the evacuating tube 26, and a tube-like resilient end piece 34 is stretched over the evacuating tube 26 at end 28 and is clamped thereto. The end piece 34, which may be a short length of bicycle tubing, extends along the boot 32 and is drawn between the boot 32 and the interior of the evacuating tube 26 as the boot is further inserted into the evacuating tube, as illustrated in FIG. 4. The end piece then provides a vacuum seal between the boot 32 and the evacuating tube 26.

Intermediate the ends 28 and 30 of the evacuating tube 26 is a fitting 40 which may be connected to a pump (not shown) for a drawing a vacuum within the tube 26. The purpose of this vacuum is to expand the boot 32 tightly against the interior surface of the tube 26 so that the element assembly 10 can be easily inserted therein, as illustrated in FIG. 5. When the vacuum is drawn, that portion of the boot within the tube 26 will have an internal diameter which is slightly larger than the external diameter of the element assembly 10.

METHOD OF ASSEMBLY The method of assemblying the hydrophone 20 comprises the following steps:

I. Coating the elongated element assembly 10 with a silicone grease such as di-methyl polysiloxane oil and silicone oxide;

2. Inserting the boot 32 into the evacuating tube 26 from its open end 28, as illustrated in FIG. 3, past the fitting 40.

3. Drawing a vacuum on fitting 40 inside the tube 26 for expanding the boot 32 tightly against the inner surface of the tube as illustrated in FIG. 4;

4. Inserting the coated element assembly 10 into the expanded boot 32, as illustrated in FIG.

5. Wiping the silicone grease smooth on the element assembly as the assembly is inserted into the tube, as illustrated in FIG. 5;

6. Releasing the vacuum inside the tube 26 through the fitting 40, as illustrated in FIG. 6;

7. Withdrawing the element assembly 10 and the boot 32 from the tube 26, as illustrated in FIG. 7. This combination is then banded with bands 24 to make the hydrophone illustrated in FIG. 8.

The silicone grease wiping process may be accomplished by a short extension 44 of the boot 32 from the open end 28 of the tube 26. The boot has an internal diameter which is slightly smaller than the external diameter of the element assembly 10. Since the short extension 44 is not subjected to the stretching action of the vacuum within the tube 26 it retains its smaller diameter to wipe the silicon grease on the element assembly 10. Accordingly, as the element assembly 10 is inserted within the boot 32a very thin film of silicone grease is presented as an acoustic couplant between the assembly 10 and the boot 32.

It is now readily apparent that the present invention provides a significantly superior hydrophone which has a greater life than prior art hydrophones utilizing castor oil as an acoustic couplant. By the use of silicone grease there is no deterioration and no absorption within the boot. This results in a reliable acoustic couplant which significantly extends the life of Navy hydrophones. The method of construction ensures a thin film of the silicone grease as an acoustic couplant without any measure of air bubbles. The silicone grease may also be used with active transducers.

What is claimed is:

1. A method of assembling a hydrophone comprising the steps of:

coating an elongated element assembly with silicon grease;

inserting an elongated resilient boot into an evacuating tube which is open at one end and closed at an opposite end;

drawing a vacuum inside the tube at a position intermediate its ends for expanding the boot tightly against the inner surface of the tube;

inserting the coated element assembly into the expanded boot;

wiping the silicon grease smooth on the element assembly as the assembly is inserted into the tube; releasing the vacuum inside the tube; and withdrawing the element assembly and boot from the tube.

2. A method as claimed in claim 1 including:

said grease being a combination of polysiloxane oil and silicone oxide.

3. A method as claimed in claim 1 including:

stopping the insertion of the boot into the evacuating tube just short of the open end of the tube.

4. A method as claimed in claim 1 including:

mounting a resilient end piece, which has an opening slightly smaller than the external diameter of the boot for sealing the boot to the evacuating tube.

5. A method as claimed in claim 4 including:

stopping the insertion of the boot into the evacuating tube just short of the open end of the tube.

6. A method as claimed in claim 5 including:

said grease being a combination of polysiloxane oil and silicone oxide. 

2. A method as claimed in claim 1 including: said grease being a combination of polysiloxane oil and silicone oxide.
 3. A method as claimed in claim 1 including: stopping the insertion of the boot into the evacuating tube just short of the open end of the tube.
 4. A method as claimed in claim 1 including: mounting a resilient end piece, which has an opening slightly smaller than the external diameter of the boot for sealing the boot to the evacuating tube.
 5. A method as claimed in claim 4 including: stopping the insertion of the boot into the evacuating tube just short of the open end of the tube.
 6. A method as claimed in claim 5 including: said grease being a combination of polysiloxane oil and silicone oxide. 